A common interface (CI) standard is commonly used in digital television (DTV) or digital video broadcast (DVB) set-top-box (STB) designs. The CI standard was created out of a need to implement a conditional access for a transport stream between a receiving channel and a source decoder on a module external to the source decoder. The conditional access allows DVB-based services to be “pay” type services or include at least some elements that are not freely available to the public at large. The CI standard supports insertion of one or more PC Card modules into the transport stream for descrambling a scrambled signal.
Referring to FIG. 1, a block diagram of a conventional STB 30 is shown. The STB 30 has a host circuit 32 including a tuner 34, a channel demodulator 36 and a source decoder 38 for converting a DVB carrier signal (i.e., RF) into a baseband signal (i.e., BBS). A PC Card module 40 can be incorporated into the STB 30 to provide a descrambling function in a descrambler circuit 41. Security codes and/or keys may be stored in a Smart Card 42 inserted into the PC Card module 40.
A scrambled transport stream (i.e., TS1) is provided from the host circuit 32 through an interface circuit 43 to the module 40 to transfer scrambled data. A descrambled transport stream (i.e., TS2) is provided from the module 40 through the interface circuit 43 to the host circuit 32 to transfer descrambled data. The CI standard defines interfaces for the transport streams TS1 and TS2 and also a command interface (i.e., CMD) between a microprocessor 44 in the host circuit 32 and another microprocessor, programmable logic, memory or logic circuit 46 in the PC Card module 40.
The CI standard defines an 8-bit wide parallel data signal. In the STB 30, eleven (11) or twelve (12) wires are conventionally used to implement each transport stream. At least 22 pins are required by the CI standard for communication where the host circuit 32 integrates the demodulator functionality and the source decoder functionality. The host circuit 32 requires eleven (11) pins for the outgoing scrambled transport stream TS1 and eleven (11) more pins for the incoming descrambled transport stream TS2. Up to two (2) additional pins are used if the host circuit 32 presents and/or receives external error signals (not shown) as part of the transport streams TS1 and TS2. When the demodulator and the source decoder functionality are implemented in a single integrated circuit package (as indicated by a dotted box 48), the 22 to 24-pin requirement to support the transport streams TS1 and TS2 can be a limitation for the integrated circuit package 48.